1. Field of the Invention
The invention relates to radar antennas and, more particularly, to a horn antenna for a radar device comprising a metal body containing a tubular hollow waveguide section which opens into a hollow horn section, a dielectric filling body filling up the inner space of the horn section, and a dielectric cover which is provided surrounding the metal body and covering the filling body at the aperture of the horn section as a protective covering for the horn antenna.
2. Description of the Related Art
FIG. 7 of U.S. Pat. No. 6,661,389 discloses a conventional horn antenna.
In general, microwave pulses, which have been generated by High Frequency (HF) energy coupled in, are radiated by a horn antenna, which also is known as cone antenna. In a combined transmitting and receiving system of a level measuring device equipped with such an antenna, the pulses reflected by a filling product are detected, and the distance from the filling product is assessed by measuring the transit time of these pulses. Radar-based level measuring devices are, for example, used for a continuous level measurement of fluids and/or bulk goods, or a combination of such products.
For antennas that are not exposed to a heavy chemical load, metallic horns or cones preferably of stainless steel are used. For highly aggressive process environments or in applications in which the filling product to be measured is, for purity reasons, not allowed to come in contact with metal, it is known to provide the metallic horn antenna with a protective layer that is corrosion-proof and permeable to microwaves.
FIG. 7 of U.S. Pat. No. 6,661,389 shows a horn antenna comprising a metal body, preferably of aluminum, in which a tubular waveguide section and an adjoining cone-like horn section are formed. The inner space of the horn section is filled with a conical dielectric filling body having a step in the zone of the transition point from the horn section into the tubular waveguide section, so that the tip of the conical filling body presents a slightly different angle with respect to the symmetry axis than the rest of its envelope surface. The metal body and the therein introduced dielectric filling body are completely enclosed by a dielectric cover, here modified polytetrafluoroethylene (PTFE). On the radiation surface where the cover is arranged over the filling body, the cover forms a convex microwave lens. In a portion remote from the radiation surface, the cover is surrounded by a sleeve of synthetic material, which is sealed with the cover by an O-ring. The sleeve is provided with an outer mounting thread so that the entire horn antenna can be screwed into an opening of a flange or vessel.
The problem of different thermal expansions of the hollow horn section and the dielectric filling body is not addressed with respect to the embodiment depicted in FIG. 7 of U.S. Pat. No. 6,661,389.
In FIG. 8, U.S. Pat. No. 6,661,389 additionally discloses another horn antenna where the metal body is screwed in the opening of a mounting flange of a vessel, where the aperture of the horn section is flush with the opening. Here, the dielectric filling body is assembled from three different parts, one of them is formed as a disk that covers and seals the opening against the environment inside the vessel. The other parts are formed as a truncated cone and a pointed cone, where the pointed cone features such an outer dimension that between its outer wall and the inner surface of the horn section a minimal gap remains. As a result, it is possible to proided compensation for expansion variations conditioned by temperature influences.
U.S. 2009/0212996 A1 discloses a horn antenna similar to that aforementioned described conventional horn antennas, with the difference that the dielectric filling body is integrally formed. Here, the dielectric filling body has a cylindrical section that is inserted in the tubular waveguide section and fixed there by a sealing and locking element, thus preventing the filling body from falling out of the horn section of the horn antenna. As the dielectric material of the filling body has a higher coefficient of thermal expansion than the metal body, a circumferential gap is provided between the outer surface of the dielectric filling body and the inner surface of the horn section. An alternative or supplemental sealing and locking element between the filling body and the metal body may be provided in the region of the aperture of the horn section.
The major drawback of the conventional horn antennas disclosed in the U.S. 2009/0212996 A1 and FIG. 8 of U.S. Pat. No. 6,661,389 is that each of these disclosed horn antennas do not protrude into the vessel so that reflections from the mounting flange or the top of the vessel may interfere with the wanted echo from the filling product in the vessel.
The conventional horn antenna depicted in FIG. 7 of U.S. Pat. No. 6,661,389 has the problem in that the hollow horn section and the dielectric filling body have different thermal expansions. The known antenna further shows a two-part design on the process side that may cause sealing and cleaning problems.